Osteoporotic fractures are increasing at an alarming rate in this country and result in over 13 billion dollars in health costs annually. Peak bone mass, that is, an individual s maximum bone mass at the completion of skeletal acquisition, is an important determinant Of fracture risk. Bone mass during adult life reflects the amount acquired during growth minus that which is subsequently lost. Thus, maximizing peak bone mass may provide an effective strategy toward preventing osteopenia and osteoporosis. It is postulated that increasing bone mass by 3-5 percent would reduce fracture risk by 20-30 percent. Our data in collegiate female gymnasts demonstrate hip and a spine bone mineral density values of up to 40 percent above normal age-matched controls and above elite runners, despite menstrual irregularities. Further, we have observed the dynamic response of bone to high impact forces in gymnasts over the training season as bone increases 2-5 percent. We plan a randomized, controlled exercise intervention to evaluate the effect of high impact loading as a means to increase bone mass during development and will determine bone mass accrual and bone geometry at the lumbar spine and proximal femur in pre-pubescent girls and boys. Further, we propose to evaluate the bone response from withdrawal of the stimulus over 6 months. Two hundred pre-pubescent children will be recruited during two separate years and randomly assigned to a jumping or a stretching group. The jumping group will perform double leg jumps and th stretching group will act as a control. Outcome variables include bone density (BMD) at the spine and hip, estimated bone volumetric density at the spine, and cross-sectional geometry of the femoral neck and diaphysis. Our proposal specifically addresses objectives related to physical activity and exercise that are outlined in the RFA. Implementing a specific bone loading program during childhood will potentially allow the bone to increase both its mass and mineralization at an earlier age and therefore provide a lager foundation of mineralization for further growth throughout adolescence until skeletal maturity is reached. Our findings are expected to provide a basis for the design of strategies to build bone during growth and thereby reduce osteoporotic fractures.